Field of the Invention
The present invention relates to a colorimetry apparatus and an image forming apparatus.
Description of the Related Art
In recent years, a color image forming apparatus such as a color printer and a color copier is required to output images with higher image quality. In particular, stability of image gradation and image color significantly affects the image quality. However, change of environment such as temperature or humidity, or long-term use of the color printer causes change of tint of an obtained image. Therefore, in order to realize stable tint, it is necessary to detect the tint of the image by using a colorimetry sensor and to provide feedback for a process condition of the image forming apparatus.
Conventionally, as one of devices for measuring tint (chromaticity) of a color of a printed matter or an object, there is used a colorimetry device. As a general colorimetry device, there is a filter type colorimetry device in which white color light is emitted to a detected object, and reflection light is received by a light receiving sensor through an RGB color filter so as to measure intensity of each color component. In addition, there is known a spectral colorimetry device in which wavelength dispersion of the reflection light is performed by using a diffraction grating, a prism, or the like, then intensity is detected for each wavelength by a line sensor, and spectral reflectance of the detected object is determined through calculation considering a wavelength distribution of the detected dispersed light, a wavelength distribution of light from a light source, spectral sensitivity of the sensor, and the like. The spectral colorimetry device is advantageous in view of the accuracy in colorimetry, and hence the spectral colorimetry device is used in many cases as a colorimetry sensor that is used for control for stabilizing tint of a color printer. Japanese Patent Application Laid-Open No. 2009-008471 proposes a spectral colorimetry device that can measure paper color information with high accuracy even if the paper sheet is conveyed in a fluttering state. In addition, Japanese Patent Application Laid-Open No. 2010-211055 proposes an image forming apparatus that uses the spectral colorimetry device to control a fixing condition with high accuracy even if a toner adhering amount fluctuates.
However, the conventional spectral colorimetry device has the following problems.
First, the spectral colorimetry device has high accuracy in colorimetry, but cost thereof is high. In particular, if a distance from the light source to the detected object is long, it is necessary to increase a light emission amount in order to secure intensity of light emitted to the detected object. As a result, there are risks such as increase of cost for a circuit of supplying a larger current and increase of cost of a light emission element. In addition, as described above, if the distance from the light source to the detected object is long, a size of the spectral colorimetry device is increased. In this case, a size of the image forming apparatus itself including the colorimetry device is increased, and hence cost of the entire apparatus is increased.
In addition, in the spectral colorimetry device, the wavelength dispersion of the reflection light is performed by using the diffraction grating, the prism, or the like, and then the intensity is detected for each wavelength by the line sensor. Therefore, if a position of the line sensor is fluctuated by thermal deformation of the spectral colorimetry device, detection accuracy may be lowered. Here, a housing of the spectral colorimetry device is usually made of a mold resin in view of easiness of molding, cost, weight, and the like. Therefore, if an ambient temperature rises during a period after the image forming apparatus is produced until the image forming apparatus is delivered to a user, the temperature of the housing of the spectral colorimetry device also rises. As a result, even after returning to room temperature, the thermal deformation (creep) of the device may be fixed. In addition, if the image forming apparatus is an electrophotographic image forming apparatus, heat generated in a fixing process may be transferred to the spectral colorimetry device so that thermal deformation is caused. A positional fluctuation of the line sensor due to the thermal deformation is insignificant, but the positional fluctuation may affect detection accuracy of the spectral colorimetry device having a structure in which the wavelength dispersion of the reflection light is performed by using the diffraction grating, the prism, or the like, and then the light is detected by the line sensor.
In addition, in order to arrange the spectral colorimetry device in the image forming apparatus, it is necessary to secure a space for the spectral colorimetry device as a matter of course. However, if the spectral colorimetry device is large in size, it is necessary to increase also a size of the image forming apparatus, which may degrade a commercial value of the image forming apparatus. Therefore, it is preferred that the spectral colorimetry device have a smaller size.
However, the spectral colorimetry device needs the light source, the diffraction grating or the prism, the line sensor, and the like, and further needs an optical guide member for guiding the reflection light to the diffraction grating or the prism, the line sensor, and the like, and a lens or the like for collimating light. Further, in order that the light subjected to the wavelength dispersion by the diffraction grating or the prism can be appropriately detected by the line sensor, the spectral colorimetry device needs a certain extent of an optical path length, which is one of factors that prevent downsizing of the spectral colorimetry device.